Chemical conversion treatment of a steel member is a chemical reaction of steel with a kind of a corrosive solution to form an adherent coating of a corrosion product on the surface of the steel member. Depending on the type of the corrosive solution which is used, chemical conversion treatment includes phosphate treatment, chromate treatment, oxalate treatment, and the like. Among others, phosphate chemical conversion treatment (also called phosphate treatment or phosphating) is widely used in the automotive industry to form a substratum coating for surface preparation prior to electrodeposition coating of a steel sheet.
In a steel sheet for an automobile, phosphate chemical conversion treatment is conducted for surface preparation prior to paint coating in order to increase the adhesion of an electrodeposited paint coating. It is desired for this treatment to form a dense phosphate coating composed of fine crystal grains. In order to ensure that such a phosphate coating is formed by phosphate chemical conversion treatment, it is known that a steel sheet is subjected, prior to the treatment, to surface conditioning using a conditioning solution containing phosphate ions and alkali metal ions.
For example, JP-A 57-82478 (Document 1), JP-A 10-245685 (Document 2), and JP-A 2000-96256 (Document 3) disclose that a dense chemical conversion coating having extremely fine crystal grains can be formed by treating a steel material with a surface conditioning solution containing a mixture of “an alkali metal phosphate as a main component and a small amount of a titanium compound and a chlorate”, “fine phosphate particles and an alkali metal salt, an ammonium salt, or the like”, or “fine phosphate particles and an accelerator (organic compound)”, respectively, followed by chemical conversion treatment with a phosphate solution (phosphating treatment).
The purpose of each of these surface conditioning techniques resides in densification and refinement of a phosphate coating which is formed by phosphating, and the surface conditioning solution itself contains both alkali metal ions and phosphate ions.
OCTG such as tubing and casing which are used when excavating oil wells are generally connected together by threaded joints. The depth of oil wells is usually 2,000-3,000 meters, but in recent years, it has sometimes reached 8,000-10,000 meters in deep oil wells for offshore oil fields and the like.
When they are placed in the environment of its use, such threaded joints connecting OCTG continue to receive the action of compound pressures including axial tensile forces resulting from the weight of the OCTG and joints themselves and internal and external surface pressures as well as underground heat. Therefore, the joints are required to maintain gas tightness and liquid tightness without breaking even in such environments. At the time of lowering tubing and casing into a well, there are cases in which a joint which has been tightened is loosened and then retightened. According to API (American Petroleum Institute), it is required that gas tightness and liquid tightness be maintained without the occurrence of galling, which is severe seizing which cannot be repaired, even when tightening (makeup) and loosening (breakout) are repeated 10 times for a joint for tubing or 3 times for a joint for casing.
A typical threaded joint for OCTG has a pin-box structure capable of forming a metal-to-metal contact seal. In such a joint, a male thread is formed on the end of an oil well pipe to form a pin, a female thread is formed on the inner surface of a threaded connecting member (a coupling) to form a box, and an unthreaded metal contact portion is provided at the end of the pin and in a corresponding position on the box. By connecting the two members, the unthreaded metal contact portions of the pin and the box contact each other and form a metal-to-metal contact seal. At the time of tightening, a liquid lubricant, which is referred to as a compound grease, containing heavy metal powder is applied in order to improve galling resistance, gas tightness, and liquid tightness. There are also threaded joints which do not need a coupling and which provide a male thread and an unthreaded metal contact portion on one end of a steel pipe to form a pin and provide a female thread and an unthreaded metal contact portion on the other end of the pipe to form a box.
The threaded portions and the unthreaded metal contact portions of a threaded joint are sometimes subjected to phosphate chemical conversion treatment and particularly manganese phosphate chemical conversion treatment, primarily with the object of improving their ability to retain the compound grease thereon and thus improving slip properties (galling resistance) and gas and liquid tightness of the joint. However, if the above-described techniques for phosphate chemical conversion treatment which was developed for surface preparation of a steel sheet for automobiles prior to paint coating and for the surface conditioning to be performed prior to phosphate treatment are applied without modification, it may not be possible to achieve the above object.
There have been a number of proposals concerning phosphate chemical conversion treatment for improving galling resistance of a threaded joint for OCTG.
For example, JP-A 5-117870 (Document 4) discloses that galling resistance and wear resistance are improved by forming surface irregularities with an average roughness of 20-60 micrometers on the surface of a joint for OCTG before the surface is subjected to phosphate chemical conversion treatment.
JP-A 2001-335956 (Document 5) discloses, following standard surface conditioning or surface roughening, performing chemical conversion treatment on the surface of a joint for OCTG of a Cr-containing steel, using a phosphate chemical conversion treating solution having a total acid number, a free acid number, and an acid ratio adjusted to be within a prescribed range. The manganese phosphate chemical conversion coating which is formed is dense with fine crystal grains.
JP-A 60-121385 (Document 6), JP-A 6-346988 (Document 7), and JP-A 7-139665 (Document 8) disclose that galling resistance of a threaded joint for OCTG made of a high chromium stainless steel having a Cr content of at least approximately 10 mass percent can be increased by “forming an Fe plated coating which may contain dispersed particles and then forming a phosphate coating”, “forming a nitride layer and then forming an anti-galling coating (manganese phosphate or Zn or Sn plated coating)”, or “forming a plating layer of iron or an iron alloy, and then forming a manganese phosphate chemical conversion coating”, respectively.
JP-A JP-A 8-103724 (Document 9) and JP-A 8-105582 (Document 10) disclose that improvement of galling resistance is achieved by forming a manganese phosphate chemical conversion coating or a nitride layer and a manganese phosphate chemical conversion coating on the threaded portions and metal-to-metal contact seal portions of a threaded joint for steel pipes and then forming an overlying resin coating containing a solid lubricant (a solid lubricant coating).
JP-B 5-40034 (Document 11) discloses that a joint for steel pipes having excellent galling resistance, wear resistance, durability, and the like is obtained, without carrying out surface conditioning, by performing chemical conversion treatment using a manganese phosphating solution to which fluoride ions have been added, thereby forming a phosphate chemical conversion coating having coarse crystal grains (20-50 micrometers) on the surface of the threaded joint.
JP-A 2003-231974 (Document 12) discloses that a chemical conversion coating having high adhesion can be formed on a threaded joint for OCTG made of a Cr-containing steel by performing chemical conversion treatment, without performing surface conditioning, using a zinc or phosphate chemical conversion treating solution containing a prescribed amount of a potassium salt to form a phosphate chemical conversion coating containing potassium and that this chemical conversion coating is dense with fine crystal grains.
Document 1: JP-A 57-82478 (1982)
Document 2: JP-A 10-245685 (1998)
Document 3: JP-A 2000-96256 (2000)
Document 4: JP-A 5-117870 (1993)
Document 5: JP-A 2001-335956 (2001)
Document 6: JP-A 60-121385 (1985)
Document 7: JP-A 6-346988 (1994)
Document 8: JP-A 7-139665 (1995)
Document 9: JP-A 8-103724 (1996)
Document 10: JP-A 8-105582 (1996)
Document 11: JP-B 5-40034 (1993)
Document 12: JP-A 2003-231974 (2003)